Inorganic microspheres are products known particularly as fillers in certain composite materials, in particular as fillers in synthetic resins or concretes. For these uses, the inorganic microspheres ordinarily consist of a soda lime silica glass of the type of those conventionally utilized for the production of glazings, for example. These materials are suitable for treatment ending in the production of microspheres which exhibit a set of satisfactory qualities.
However, for certain specific applications, it is necessary to resort to microspheres exhibiting a refractory character which is greater than that of conventional glass microspheres.
Production of conventional glass microspheres is performed industrially by subjecting glass particles to a heat treatment followed by an instantaneous cooling. Gases are released from the glass composition by the high temperatures utilized to assure the expansion of the particles. During the cooling step which follows, the molten glass hollow spheres are immediately solidified to prevent collapse thereof. This treatment is performed, for example, by passing the glass particles through the flame of a burner operating at high temperatures, followed by cooling with large volumes of ambient air.
This technique is possible because of the relatively low softening and melting temperatures of conventional glasses. For glass particles with a high silica content (more than 90% by weight), the temperature for which a satisfactory viscosity for the formation of spheres is reached, a viscosity whose logarithm is close to 2, is close to 1700.degree. C. and never less than about 1600.degree. C. Under these conditions, a treatment of the conventional type for the production of glass microspheres is not possible.
To get around this difficulty, the techniques previously proposed for the production of microspheres with a high silica content include the production of a borosilicate gel. This technique has been described many times, particularly in U.S. Pat. No. 4,336,338. Production of microspheres according to this technique comprises the formation of a borosilicate gel in an aqueous medium with various solvents, drying of the gel, grinding and a relatively complex heat treatment by making the particles pass in a furnace exhibiting several successive zones kept at different temperatures. Thus, such a process is a relatively delicate, complex and expensive mode of production.
The invention proposes a new technique of production of microspheres with a high silica content which avoids the passage through a gel phase and the resulting drawbacks.